A. Technical Field
The present invention relates generally to power control within an optical network, and more particularly, to power detection on a semiconductor optical amplifier, and in response, adjustment of a power saturation level on the semiconductor optical amplifier.
B. Background of the Invention
Service providers are experiencing an ever-increasing demand for bandwidth fueled by Internet access, voice, data, and video transmissions and this demand for bandwidth will likely continue to grow. Due to this demand, network capacities are being stretched to their limit. As a result, there has been an increasing effort to lay fiber in order to expand the capacity of existing networks and build new higher capacity networks.
Optical amplifiers, which boost the power of optical signals within these networks, are a basic building block for many types of optical systems. Fiber optic communications systems typically require the use of optical amplifiers in order to properly carry optical information within a network. A typical communications system includes a transmitter connected to a receiver via fiber. The transmitter incorporates information into an optical signal and transmits the optical signal via the optical fiber to the receiver. The receiver recovers the original information from the received optical signal. In these systems, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers are used to compensate for these attenuations and degradations by amplifying the optimal signal to an appropriate power level. Moreover, receivers typically operate properly only within a narrow range of optical signal power levels; and optical amplifiers are used to boost an optical signal to the proper power range for the receiver.
Networks may place different power output requirements on optical amplifiers. For example, an OC-192 network having 48 channels requires a higher amplifier power output than an OC-3 network containing 12 channels. This difference is caused by the increased rate of the OC-192 network and the larger number of channels within the network. In the new switched metro WDM links with Gb/s data channels, it is important that optical amplifiers within the network do not saturate. Saturation occurs when an optical amplifier is unable to sufficiently amplify an optical signal to a desired power level that is above the amplifier""s power saturation level. This power saturation level defines an output power ceiling above which the optical amplifier does not operate properly.
The power saturation level of a semiconductor optical amplifier (SOA) is defined by a number of factors including the characteristics of the active region operating within the SOA and the rate at which the active region is pumped. Saturation occurs when an active region within an SOA is unable to effectively amplify at least one channel within the optical signal. This failure is caused by insufficient power within the active region to amplify each channel to a proper power level resulting in one or more channels not being properly amplified. This phenomenon is called crosstalk and may degrade a signal, or a channel within the signal, to the point of not being readable at a receiver within the network. Channel crosstalk is an important consideration that needs to be addressed when optical networks are designed or scaled.
In order to effectively scale existing optical networks, a service provider should be able to control power levels on signals traveling across a network. As the number of channels expands and/or the data rate increases within a network, the power output requirement of the amplifiers within the network will also likely increase. This increase is caused by the fact that the required gain applied to each channel remains constant while the number of channels within the optical signal increases. Accordingly, network managers typically address this issue in a number of different ways when a network is scaled. First, a network manager may replace some or all of the amplifiers within amplifiers having higher output power. Second, a network manager may insert additional amplifiers within the network. Third, a network manager may increase the output power of existing amplifiers within the network.
SOAs allow network managers to adjust output power because they typically may have a tunable pump source. As previously mentioned, in practice, the output power of an SOA depends, at least in part, on the rate at which the active region is pumped. Accordingly, network managers may increase an SOA power output by increasing the current pumping the active region. However, this process is currently time consuming and inaccurate because of the lack of real time data regarding power output on each of the optical amplifiers.
Power monitoring traditionally required that an optical signal be tapped in order to determine a power level. One such tapping method is the use of a power coupler positioned within a piece of fiber. This tapping process reduces the power level on the optical signal and adds more fiber components, cost, and space to increasingly complex fiber systems. Accordingly, data regarding specific power levels on each optical amplifier is generated at a power expense to the optical signal and may not be as exact as a network manager may desire. As a result, optical network designers generally try to minimize the number of taps within a network.
Accordingly, there is a need for a system and method that monitors and controls an optical amplifier output power level by adjusting the rate at which the optical amplifier is pumped. Additionally, there is a further need for this system to avoid tapping an optical signal while monitoring and controlling the optical amplifier in order to reduce power loss to the optical signal.
The present invention overcomes the deficiencies and limitations of the prior art by providing an optical amplifier with an output power monitor and control system. In particular, the present invention provides an optical power detection system that avoids the use of power couplers or taps by detecting a ballast laser signal emitted from a lasing semiconductor optical amplifier (SOA). The lasing SOA emits the ballast laser signal in response to the amplification of the optical signal. Examples of lasing SOAs include vertical lasing SOAs, horizontal lasing SOAs, and longitudinal lasing SOAs. The invention comprises an optical detector that converts the ballast laser signal to an electrical signal, a power monitor device that identifies the output power on the lasing SOA, and a tunable pump that pumps an active region within the lasing SOA.
An optical signal propagates through an amplification path within the lasing SOA causing the signal to be amplified. A laser cavity within the amplification path contains a semiconductor gain medium that is pumped above a lasing threshold for the laser cavity. As a result, lasing occurs producing laser radiation within the laser cavity. This laser radiation operates as a ballast to prevent gain saturation within the laser cavity during the amplification of the optical signal. As a result, the gain within the laser cavity is clamped causing the laser radiation to be emitted as a ballast laser signal.
This ballast laser signal corresponds to the strength of the optical signal within the laser cavity because the laser cavity gain is clamped. The present invention utilizes this relation to determine a power level of an optical signal as it propagates through a lasing SOA. In one embodiment of the invention, a detector is positioned proximate a surface on the lasing SOA that emits this ballast laser signal. For example, a detector may be integrated on the ballast laser emitting surface near the output of the lasing SOA. This detector converts the ballast laser light near the output to an electrical signal from which the output power may be determined.
A power monitor receives the electrical signal and identifies if the lasing SOA is approaching or has exceeded a power saturation level corresponding to the lasing SOA. If the power monitor determines that the lasing SOA is approaching or is currently saturated, then a pump source, coupled to the power monitor, is signaled. The pump source controls the rate at which the active region within the lasing SOA is pumped. In response to this signal from the power monitor, the pump source increases or decreases its pumping rate. This change in the pumping rate either raises or lowers the power saturation level of the lasing SOA, thereby, reducing the danger of saturating the lasing SOA. Thus, the power saturation level is dynamically increased when the lasing SOA output power approaches the power saturation level or may be decreased if the lasing SOA is not in any danger of saturating. In contrast to non-lasing SOAs, the saturation power level of the lasing SOA is directly affected and proportional to the pump current level.
The power monitor may also be used to control other optical amplifiers within a network such as a pair of SOAs coupled in series. For example, a power monitor may increase a pumping rate on a second lasing SOA in response to the above described power detection of a first lasing SOA. Accordingly, optical signal power levels may be detected on a first lasing SOA and, in response; a power saturation level may be adjusted on a second lasing SOA. Additionally, this power monitor and control may be centralized within a single unit that addresses multiple lasing SOAs within a network. This centralization allows for easier network management and control because the power saturation levels of many lasing SOAs may be adjusted at a single location.